When cutting work is performed by a machine tool, so-called “chatter vibration” is occasionally generated if the tool or the workpiece has low rigidity. Generation of chatter vibration causes a problem such as breakage of the tool or degradation of surface accuracy of the workpiece. This chatter vibration is generated due to fluctuation in cut-off thickness of the workpiece and an increase in vibration because of generation of a phase delay between undulation (vibration) caused on the machined surface before one rotation and vibration due to the current cutting. As technology for suppressing chatter vibration, measures have been proposed by JP 49-105277 A and JP 61-3522 U. The technologies described in JP 49-105277 A and JP 61-3522 U vary the rotation speed of the rotating shaft to make the force inputted to the tool irregular so as to suppress chatter vibration. JP 49-105277 A and JP 61-3522 U disclose a device that suppresses chatter vibration by setting a variable amplitude and a variable period when the rotation speed of the rotating shaft is varied.
The methods described in JP 49-105277 A and JP 61-3522 U, however, require setting of two values of a variable amplitude and a variable period to vary the rotation speed. In order to suppress chatter vibration, it is known that the effect is high when a variable period is set short at a variable amplitude of a certain value or higher. That is to say, it is known that the chatter vibration suppressing effect is high when the rotating shaft is accelerated and decelerated sharply. When the rotating shaft is repeatedly accelerated and decelerated excessively, however, input power into the motor increases, and there is a possibility of the motor produces heat and is broken.
Accordingly, there is considered a method of performing the originally intended machining after a limit value of varying the rotation speed was found from the maximum input power of the motor and the moment of inertia of the rotating body. To do so, it is necessary to perform a dedicated operation to determine the moment of inertia and to record it in an NC device for controlling the machine tool. When the same workpiece is machined, it is not necessary to update the moment of inertia, but when a different workpiece is machined, it is necessary to perform the dedicated operation to determine the moment of inertia again, or the operator needs to set the previously determined moment of inertia into the NC device, thereby increasing the number of work steps. When the same workpiece is machined by another machine tool, the same number of work steps is needed, and the process is not rational.